Aircraft have been designed for decades which enable pilots to directly control the aircraft by their instincts or their feelings, i e., "by the seat of their pants". This may have been satisfactory 80 years ago but, with the advent of highly powered and responsive jet fighters, instincts are no longer enough. Now many jet fighters, such as the F-16, have sufficient power to attempt a maneuver that is violent enough to damage the aircraft or black-out the pilot if inadvertently or erroneously requested. Also, the demands on the pilot to coordinate flight controls increase with the power and speed of the aircraft. A slight error can result in the plane going out of control, sometimes irrecoverably. Additionally, with all of this aeronautical capability, more functionality is demanded for the aircraft. In some cases these new functions may consist of systems, such as an autopilot which reduces the work load of the pilot by automatically flying a designated course. It may also be necessary to provide a highly demanding pilot work load function, such as terrain-following, which maintains a designated altitude close to the ground.
In certain high performance aircraft such as the F-16 fighter plane, these functions are accomplished by analog computers. A flight control computer is used to control movement of the airplane control surfaces so as to overcome flying instabilities that result from the aircraft design. The flight control computer may also restrain or limit the commands from the pilot. An additional analog computer may provide a simple function such as an autopilot that maintains a straight and level course.
An analog computer is a series of hard-wired circuits that cause an output signal to vary continuously in a predefined manner in response to one or more input signals. In contrast, a digital computer operates on discrete numbers to generate a sequence of numbers that can define an output signal. Whereas an analog computer must be specially designed and wired for each different relationship between its input and output signals, this relationship is defined in a digital computer by a program that can more easily be changed. Not only is a digital computer more flexible than an analog computer, but a special circuit need not be designed and built for each function that is to be performed. It thus becomes practical to implement more complex functions with a digital computer.
For these reasons, digital computers have generally replaced analog computers in the newest aircraft designs. Nevertheless, there remains a large base of operating aircraft that have analog flight control computers. It would be desirable to provide the flexibility and functionality of digital computers to these aircraft but the cost of replacing the analog computers would be beyond the economic constraints of most operators.
In more modern designs, the aircraft stability is controlled by a digital flight control computer while a mission computer controls multiple complex functions, such as terrain-following and automatic landing systems. This type of digital system is designed into many new aircraft and supersedes the old all-analog approach.
Analog and digital computer systems are inherently incompatible with each other due to the different techniques of communicating information. It is thus not practical to upgrade the functionality of an older aircraft by simply connecting a modern digital computer in place of an existing analog flight control computer. Generally, an upgrade to a digital computer would require major rewiring of an aircraft. Thus, it would be extremely costly to replace the existing analog computers with digital computers. The large inventory of expensive and potentially highly capable aircraft add the emerging power of avionics systems makes it imperative to find a way to economically bring the capabilities of modern digital avionics to existing all-analog aircraft. This invention provides the connection between analog and digital technologies by establishing an interface between new digital mission computers and the analog flight control computer. This invention enables the latest functionality of a digital flight system to be added to existing aircraft while minimizing the conversion costs and minimizing flight requalification on the aircraft since the analog flight control system is maintained with this invention.